1. Field of the Invention
The present invention relates to a positive electrode for a secondary battery, a secondary battery, and a method for fabricating the positive electrode for a secondary battery.
2. Description of the Related Art
With the recent rapid spread of portable electronic devices such as cell phones, smartphones, electronic book readers (e-book readers), and portable game machines, research and development have been extensively conducted on secondary batteries that are power sources for driving the portable electronic devices, typified by lithium secondary batteries. The secondary batteries are of growing importance in a variety of uses; for example, hybrid vehicles and electric vehicles receive attention as a measure against global environmental problems and oil resources problems.
A lithium secondary battery, which is one of the secondary batteries and widely used because of its high energy density, includes a positive electrode including an active material capable of occlusion and release of lithium, such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), a negative electrode formed of a carbon material capable of occlusion and release of lithium, such as graphite, an electrolyte solution in which an electrolyte formed of a lithium salt such as LiBF4 or LiPF6 is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate, and the like. A lithium secondary battery is charged and discharged in such a way that lithium ions in the secondary battery move between the positive electrode and the negative electrode through the electrolyte solution and are inserted into or extracted from the active materials of the positive electrode and the negative electrode.
A binder is mixed into an electrode in order that active materials can be bound or an active material and a current collector can be bound. Since the binder is generally an organic high molecular compound such as polyvinylidene fluoride (PVDF) which has an insulating property, the electron conductivity of the binder is extremely low. For this reason, as the ratio of the mixed binder to the active material is increased, the amount of the active material in the electrode is relatively decreased, resulting in the lower charge and discharge capacity of the secondary battery.
Hence, in Patent Document 1, mixture of a conductive additive such as acetylene black (AB) or a graphite particle increases the electron conductivity between active materials or between an active material and a current collector. Thus, a positive electrode active material with high electron conductivity can be provided.
However, because acetylene black generally used as a particulate conductive additive is a high-volume particle with an average diameter of several tens of nanometers to several hundreds of nanometers, contact between acetylene black and an active material hardly becomes surface contact and tends to be point contact. Consequently, contact resistance between the active material and the conductive additive is high. Further, if the amount of the conductive additive is increased to increase contact points between the active material and the conductive additive, the proportion of the amount of the active material in the electrode decreases, resulting in a decrease in the charge and discharge capacity of the battery.
On the other hand, Patent Document 2 discloses the use of a single layer or a stacked layer of graphene (which is referred to as two-dimensional carbon in Patent Document 2) as a conductive additive, instead of the use of a particulate conductive additive such as acetylene black. Extending two-dimensionally, the single layer or the stacked layer of graphene improves the adhesion between active materials and the adhesion between conductive additives, leading to an increase in conductivity of an electrode.